1. Field of the Invention
This invention is directed to methods for extending the cycle life of solid, secondary lithium electrochemical cells and batteries.
2. State of the Art
Electrochemical cells comprising an anode, a cathode, and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid electrochemical cells". One class of solid electrochemical cells are the rechargeable (secondary) lithium cells which comprise a solid electrolyte interposed between a lithium anode and a compatible cathode suitable for recycling (recharging) the cell after discharge. The composition and method of making such electrochemical cells has been described in the patent literature, U.S. Pat. Nos. 4,830,939; 5,037,712; U.S. patent applications Ser. Nos. 07/918,438, filed Jul. 22, 1992, now U.S. Pat. No. 5,262,253; and 08/049,490, filed Apr. 19, 1993, now U.S. Pat. No. 5,300,375, each of which is incorporated herein by reference in its entirety.
A solid, secondary battery contains several solid, secondary electrochemical cells wherein the current from each of the cells is accumulated by a conventional current collector so that the total current generated by the battery is roughly the sum of the currents generated from each of the individual electrochemical cells employed in the battery. Such an arrangement enhances the overall current produced by the solid, secondary battery to levels which render such batteries commercially useful.
However, one problem encountered with the use of solid, secondary electrochemical cells in such batteries is the limited cycle life of the battery, i.e., the number of rechargings the battery can accept before the battery is no longer able to maintain acceptable levels of charge capacity. Specifically, the cycle life of the solid, secondary battery is related to the cycle lives of individual electrochemical cells comprising the battery. In general, when one of the electrochemical cells in the battery ceases to maintain acceptable levels of capacity, the battery must drain more current from the remaining electrochemical cells so as to produce the same overall level of current from the battery which results in a reduction of the capacity of the remaining electrochemical cells in the battery. This in turn results in a significant reduction in the cycle life of the cells and hence that of the battery.
Without being limited to any theory, it is believed that reduced cycle life in secondary lithium electrochemical cells containing a solid, solvent-containing electrolyte interposed between a lithium anode and a compatible cathode, arises, in pan, from lithium dendrite growth on the electrode surface during recharging of the discharged cell. Because the solid, solvent-containing electrolyte interposed between the lithium anode and the cathode is pliable, dendrite growth pushes aside this pliable electrolyte. Additionally, because dendrite growth is cumulative over repeated charging cycles, these growing dendrites will eventually contact the counter electrode resulting in microshorts in the electrochemical cell. This accumulation of microshorts eventually shorts the electrochemical cell thereby leading to termination of its cycle life for cells unable to accept charge. The situation is exacerbated by rapid charging procedures which serve to accelerate termination of the cells cycle life.
In view of the above, the an has been searching for charging methods, particularly rapid charging methods, which would prolong the cycle life of the cells and hence that of the battery.